[v2,4/5] fscrypt: only set dentry_operations on ciphertext dentries
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Message ID 20190320183913.12686-5-ebiggers@kernel.org
State Not Applicable
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Series
  • fscrypt: d_revalidate fixes and cleanups
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Commit Message

Eric Biggers March 20, 2019, 6:39 p.m. UTC
From: Eric Biggers <ebiggers@google.com>

Plaintext dentries are always valid, so only set fscrypt_d_ops on
ciphertext dentries.

Besides marginally improved performance, this allows overlayfs to use an
fscrypt-encrypted upperdir, provided that all the following are true:

    (1) The fscrypt encryption key is placed in the keyring before
	mounting overlayfs, and remains while the overlayfs is mounted.

    (2) The overlayfs workdir uses the same encryption policy.

    (3) No dentries for the ciphertext names of subdirectories have been
	created in the upperdir or workdir yet.  (Since otherwise
	d_splice_alias() will reuse the old dentry with ->d_op set.)

One potential use case is using an ephemeral encryption key to encrypt
all files created or changed by a container, so that they can be
securely erased ("crypto-shredded") after the container stops.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/crypto/hooks.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

Comments

Theodore Y. Ts'o April 17, 2019, 2:07 p.m. UTC | #1
On Wed, Mar 20, 2019 at 11:39:12AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Plaintext dentries are always valid, so only set fscrypt_d_ops on
> ciphertext dentries.
> 
> Besides marginally improved performance, this allows overlayfs to use an
> fscrypt-encrypted upperdir, provided that all the following are true:
> 
>     (1) The fscrypt encryption key is placed in the keyring before
> 	mounting overlayfs, and remains while the overlayfs is mounted.
> 
>     (2) The overlayfs workdir uses the same encryption policy.
> 
>     (3) No dentries for the ciphertext names of subdirectories have been
> 	created in the upperdir or workdir yet.  (Since otherwise
> 	d_splice_alias() will reuse the old dentry with ->d_op set.)
> 
> One potential use case is using an ephemeral encryption key to encrypt
> all files created or changed by a container, so that they can be
> securely erased ("crypto-shredded") after the container stops.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Looks good, applied.

					- Ted

Patch
diff mbox series

diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index 2e7498a821a48..9d8910e86ee5d 100644
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -115,9 +115,8 @@  int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry)
 		spin_lock(&dentry->d_lock);
 		dentry->d_flags |= DCACHE_ENCRYPTED_NAME;
 		spin_unlock(&dentry->d_lock);
+		d_set_d_op(dentry, &fscrypt_d_ops);
 	}
-
-	d_set_d_op(dentry, &fscrypt_d_ops);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);